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Two pairs of gas giants locked in unusually tight orbital dances have been discovered around old, dying stars.

The four gas giants were discovered via the Doppler shift method, that is, by detecting the wobble in the light emitted by their host stars as the planets track around them. The systems are part of the Keck Subgiants Planet Survey, which searches for planets around stars 40 to 100 percent times larger than our Sun.

When the subgiant stars like HD 200964 and 24 Sextanis eventually become red giants they will likely engulf their planets or even fling them out of the systems completely.
Image: NASA, ESA & G Bacon (STScI).

In the case of HD 200964, located 223 light years from Earth, and 24 Sextanis, 244 light years away, the planets were found to be locked in a tight orbital embrace such that HD 200964's planets are separated by just 0.35 AU – comparable to the distance between Earth and Mars – while 24 Sextanis' brood are separated by 0.75 AU. “A planetary system with such closely spaced giant planets would be destroyed quickly if the planets weren’t doing such a well synchronized dance,” says Eric Ford of the University of Florida. “This makes it a real puzzle how the planets could have found their rhythm.”

The planets' imposing masses – all much greater than Jupiter – exert considerable gravitational influence on their siblings. For example, the gravitational tug between HD 200964's two planets is 700 times greater than that between the Earth and Moon. Furthermore, the planets are also located relatively close to their parent stars; those orbiting HD 200964 do so once every 630 and 830 days; 24 Sextanis' planets have orbital periods of 455 and 910 days.

After planets form they often migrate around their orbits in a disorderly fashion before settling down and sometimes becoming locked in a resonance. One such scenario would see a planet orbit its star twice for every one complete orbit that another planet makes – this is known as a 2:1 resonance, and is one of the most common and stable configurations.

In our own Solar System, Jupiter's moon Europa is in a 2:1 resonance with Io, while Ganymede makes one orbit every time Io makes four. Image: NASA.

“Planets tend to get stuck in the 2:1. It’s like a really big pothole,” says John Johnson at Caltech. “But if a planet is moving very fast it can pass over a 2:1. As it moves in closer [to its sun], the next step is a 5:3, then a 3:2, and then a 4:3.”

The 24 Sextanis system is in this 2:1 arrangement, while the planetary pair in HD 200964 has arrived at the 4:3 configuration. “The closest analogy in our Solar System is Titan and Hyperion, two moons of Saturn which also follow orbits synchronized in a 4:3 pattern,” says Ford. “But the planets orbiting HD 200964 interact much more strongly, since each is around 20,000 times more massive than Titan and Hyperion combined.”

“This is the tightest system that’s ever been discovered,” Johnson adds, “and we’re at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar planets, and it shows that exoplanets continuously have this ability to surprise us. Each time we think we can explain them, something else comes along.”

The so-called 'subgiants' that these host stars belong to is a class of star that has run out of hydrogen for nuclear fusion, causing its core to collapse and its outer envelope to swell.

“Right now, we’re monitoring 450 of these massive stars, and we are finding swarms of planets,” enthuses Johnson. “Around these stars, we are seeing three to four times more planets out to a distance of about three AU – the distance of our asteroid belt – than we see around main-sequence stars. Stellar mass has a huge influence on frequency of planet occurrence, because the amount of raw material available to build planets scales with the mass of the star.”

When HD 200964 and 24 Sextanis eventually become red giant stars some 10 to 100 million years down the line they will throw off their outer atmospheres, which will likely change the gravitational dynamics of the whole system, altering the planets' orbits and perhaps even flinging them out of the system completely.

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